Update broke collaboration on iCloud, how to fix?

Since one of our mac running MacOS Monterey updated automatically iWork (Pages, Keynote, Numbers to 11.2) the Macs on older devices running MacOs Catalina can't edit Numbers files anymore.

When opening a with iCloud shared file on a Mac running Catalina, we get the following message:

To collaborate on this spreadsheet, get the latest version of Numbers from the App Store or use Numbers on iCloud.com

But on the AppStore the update isn't available, because that Mac can't update to MacOS 11 Monterey cause of it's age!

So, one updating Mac broke our entire collaboration setup without a sign of a warning overnight automatically!?

Is Apple seeing this as an issue that they are trying to fix or is this their way of saying I shall buy newer Macs?

I don't mind to downgrade all iWork apps, but is that even possible?

Or is there another workaround?

Any help is very appreciated, as we need to fix this asap cause Apple just broke our work system!